The present invention relates to an image forming apparatus such as an electrophotographic apparatus, an electrostatic recording apparatus or the like which produces images through an image forming process including a step of electrically charging an image bearing member such as a photosensitive member, a dielectric member or the like.
The image forming apparatus such as an electrophotographic apparatus requires an electric charging step of charging the image bearing member uniformly to a predetermined potential in order to form an electrostatic latent image on the image bearing member. For this purpose, a non-contact type corona charger or the like has been used as a means for the charging. However, the corona charger produces ozone and requires such a high voltage as approx. 10 KV has to be applied between the charging device and the image bearing member.
Recently, a charging means has been proposed to avoid these problems. In such a means, a charge member is directly contacted to the image bearing member and is supplied with a voltage by which the image bearing member is charged uniformly (so-called contact charging device).
A) Charging Roller
A typical contact charging device is a charging roller 2-X as shown in FIG. 6.
In the charging roller, the charge member 2-X-a is in the form of a roller (charging roller) having an electroconductive base roller and a surface layer of intermediate resistance layer. The charging roller 2-X-a is contacted to the image bearing member 1 at a predetermined pressure and is rotatably supported on bearings. It is rotated in the direction indicated by arrow b by rotation of the image bearing member 1 which is rotated in the direction indicated by an arrow a. Between the charging roller 2-X-a and the image bearing member 1, a predetermined charging bias voltage is applied from a voltage source S1 so that said image bearing member 1 is uniformly charged to a predetermined potential.
Here, the voltage applied to the roller may be (1) a DC voltage only or (2) a DC voltage biased with an AC voltage.
In the case of (1), in order to charge the image bearing member 1 to a potential of xe2x88x92600 V, the applied voltage is approx. xe2x88x921300 V, and in the case of (2), the applied DC voltage is xe2x88x92600 V and the AC voltage is not less than 1500 Vpp.
The charging mechanism in these cases is based on the Paschen""s law, and an electric discharge phenomenon arises in a region satisfying the Paschen""s law in which the distance between the charging roller 2-X-a and the image bearing member 1 is within a predetermined range (region H in FIG. 6).
However, as will be understood from the charging mechanism, the contact charging device of this type creates the discharge which is the same as with the corona charger within a fine space region H, and therefore, the ozone is produced although the amount of ozone production is remarkably smaller than with the corona charger. The ozone produces oxide nitrogen, and if it is deposited on the image bearing member 1, an image defect is produced due to the low resistance of the deposited matter.
B) Injection Charging Device
This injection charging process system is proposed in Japanese Laid-open Patent Application Hei 6-3921 which is free of such a problem of ozone generation, and therefore, the voltage applied to the charging device can be further reduced.
The feature of the charging process is that surface potential of the charged image bearing member is substantially the same as the voltage applied to the charging device. This system does not use the electric discharge phenomenon, and charge injection occurs into the image bearing member by the transfer of electric charges between the surface of the image bearing member and the charge member contacted thereto.
(1) Magnetic Brush Charging Apparatus
In order to embody the injection charging process, some types of injection charging devices have been proposed.
A typical example is a magnetic brush type charging device 2-Y as shown in FIG. 7. The charging device comprises a magnet 2-Y-a, a non-magnetic charging sleeve 2-Y-b containing the magnet 2-Y-a therein, a magnet carrier (magnetic carrier, magnetic powder member) 2-Y-c, an electroconductive regulating blade 2-Y-d and a housing 2-Y-e and so on.
The magnet carrier 2-Y-c is made of magnetic material (particles) which is electroconductive.
The charging sleeve 2-Y-b is disposed in the housing 2-Y-e and is rotatable, and a part of the peripheral surface thereof is exposed to the outside through an opening of the housing. In the charging device 2-Y, the exposed portions of the charging sleeve 2-Y-b is faced to the image bearing member 1 with a predetermined small gap therebetween. The magnet 2-Y-a is not rotatable The magnet carrier 2-Y-c is retained in the housing 2-Y-e. A regulating blade 2-Y-d is provided in the opening of the housing 2-Y-e and provides a predetermined gap between the regulating blade 2-Y-d and the charging sleeve 2-Y-b. 
The magnet carrier 2-Y-c in the housing 2-Y-e is magnetically attracted and retained in the form of a magnetic brush on the peripheral surface of the charging sleeve 2-Y-b by the magnetic field generated by the magnet 2-Y-a, and is fed by the rotation of the charging sleeve 2-Y-b. The layer thickness thereof is regulated to a predetermined thickness by the regulating blade 2-Y-d, and the layer is carried to the outside of the opening of the housing 2-Y-e to be brought into contact to the surface of the image bearing member 1. It rubs the surface of the image bearing member and returns into the housing 2-Y-e with the continuing rotation of the charging sleeve 2-Y-b. 
The image bearing member 1 is rotated in the direction indicated by an arrow a, and the charging sleeve 2-Y-b is rotated in the direction indicated by an arrow S which is opposite from the rotational direction of the image bearing member 1 at the contact portion (charge portion) between the image bearing member 1 and the magnetic brush of the magnet carrier 2-Y-c. Thus, there is provided a peripheral speed difference between the magnetic brush of the magnet carrier 2-Y-c and the image bearing member 1 so that magnetic brush rubs in the surface of image bearing member 1 with the rotation of the charging sleeve 2-Y-b. 
In the magnetic brush charging apparatus 2-Y of this example, the regulating blade 2-Y-d is supplied with a DC voltage of xe2x88x92600 V for example as a charging bias voltage from the voltage source S1. Therefore, the portion of the image bearing member 1 to which the magnetic brush of the magnet carrier 2-Y-c is contacted tends to acquire the same potential. This time, if the charge is injected from the magnet carrier 2-Y-c into the image bearing member 1 beyond an energy barrier at the surface of the image bearing member 1, then the image bearing member 1 is electrically charged. If it cannot be injected beyond the energy barrier or if the charge returns to the magnet carrier 2-Y-c when the magnet carrier 2-Y-c is brought out of contact from the image bearing member 1, then the image bearing member 1 is not charged. In the phenomenon, the energy barrier at the surface of the image bearing member 1 and a retention performance of the charge are important, and when the phenomenon is taking as a competitive reaction, the frequency of chances of contact between the magnet carrier 2-Y-c and the image bearing member 1 is important.
In order to raise the contact frequency, and the particle size of the magnet carrier 2-Y-c is reduced; the magnetic force provided by the magnet 2-Y-a is made stronger to increase the density of the magnetic brush of the magnet carrier 2-Y-c; and/or the peripheral moving direction of the charging sleeve 2-Y-b is made opposite from the peripheral moving direction of the image bearing member 1 at the charge portion to increase the relative speech between the image bearing member and the magnetic brush of the magnet carrier 2-Y-c is increased. These are effective to increase the number of contacts per unit time between the particles of the magnet carrier 2-Y-c and the image bearing member 1.
In this manner, the particles of the magnet carrier 2-Y-c which provide sites of charge injection into the image bearing member 1 can be contacted to the image bearing member at a high probability, by which the surface potential of the image bearing member 1 becomes substantially the same as xe2x88x92600 V applied to the regulating blade 2-Y-d, and a uniform charging (in a microscopic sense) is accomplished
(2) Furbrush Charging Device
An injection charging device 2-Z of a type different from the magnetic brush type may use a furbrush roller 2-Z-a as the charge member as shown in FIG. 8.
In the furbrush type, the role of the magnetic brush of the magnet carrier 2-Y-c in the magnetic brush charging apparatus 2-Y is performed by an electroconductive fur.
The furbrush roller 2-Z-a comprises electroconductive soft fur at a high density, and fur tip portions are contacted to the surface of the image bearing member 1. The image bearing member 1 is rotated in the direction indicated by an arrow a, and the furbrush roller 2-Z-a is moved in the direction indicated by an arrow s which is opposite from the moving direction of the image bearing member 1 at the contact portion (charge portion) relative to the image bearing member 1.
Namely, the furbrush roller 2-Z-a is rotated with a peripheral speed difference relative to the image bearing member 1 to rub the surface of the image bearing member 1 by the furbrush.
The furbrush roller 2-Z-a is supplied with a predetermined DC voltage from a voltage source S1 as a charging bias voltage, so that surface of the image bearing member 1 is electrically charged.
(3) Sponge Charging Roller
As a further different type charging device (different from the magnetic brush type and the furbrush type), an injection charging device 2-A using a charging sponge roller 2-A-e as the charge member, is shown in FIG. 9, has been proposed.
In this type, the charging sponge roller 2-A-a has pores on the surface thereof which is rotated in contact with the image bearing member 1, and the pores contain electroconductive particles (charging-promotion particles) having a relatively low resistance. The electroconductive particles Z correspond to the magnet carrier 2-Y-c in the magnetic brush type and function as the injection site.
The image bearing member 1 is rotated into direction indicated by an arrow a, and the charging sponge roller 2-A-a is rotated in the opposite peripheral direction which is opposite from that of the image bearing member 1 at the contact portion (charge portion) between the image bearing member 1 and the charging sponge roller 2-A-a. The charging sponge roller 2-A-a is rotated with a peripheral speed difference relative to the image bearing member 1 to rub the surface of the image bearing member 1. In this case, the electroconductive particle Z is present in the contact nip between the charging sponge roller 2-A-a and the image bearing member 1. The charging sponge roller 2-A-a is supplied with a predetermined DC voltage as the charging bias voltage from a voltage source S1, so that surface of the image bearing member 1 is electrically charged.
In the injection charging devices such as the above-described magnetic brush charging apparatus, furbrush charging device and sponge charging roller, it is important that injection sites are assured between the charge member and the image bearing member in order to provide sufficient charging power, with high-efficiency and for a long term. In the long term use, if even a small amount of developer or the like remains on the image bearing member in the form of residual toner as a result of image transfer, the developer is supplied to the charge member Most of them are collected back into the developing device. However, there exists such a developer accumulated on the charge member. The accumulation amount may be large as the case may be. If this occurs, the resistance of the charge member increases with a result of lowered injection efficiency, and therefore, the charging property is not satisfactory.
When an excessive amount of the developer is present on the charging member, the developer on the charge member is not sufficiently charged on the charge member and is easily transferred onto the image bearing member even if the charging member has sufficient charging power. Therefore, the developer is not sufficiently collected back into the developing device in the developing process and is transferred onto the transfer material with the result of image defect, as the case may be.
If the excessive amount of accumulated developer on the charge member is transferred onto the image bearing member, the developer blocks in the image explosion light in the exposure process with the result of disturbance to the latent image formation.
Even in the case that amount of the residual developer is so minimize that accumulation of the developer on the charge member is not a problem in the repetition of the normal image forming process, a problem may arise. For example, when the transfer material is jammed during output of the image, and the image forming operation is resumed with the developed image remains of the image bearing member, a great amount of the developer is deposited directly on the charge member in a cleanerless system. If such occurs repeatedly, the insufficiency of the charging power and the contamination of the transfer material and the disturbance to the latent image result.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which the charge member is efficiently cleaned.
It is another object of the present invention to provide an image forming apparatus in which an image defect attributable to the developer deposited to the charge member is avoided.
It is a further object of the present invention to provide an image forming apparatus in which the charging power of the charge member is maintained stable for a long period of time.
It is a further object of the present invention to provide an image forming apparatus in which charge member is suitable for injection charging of the image bearing member.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.